This invention relates generally to shields for electromagnetic interference, and, more particularly, to shields that perform in a manner similar to solid metal, but that include substantial amounts of lighter, non-metallic material.
Electromagnetic shields are customarily used for electrical circuitry that must be operated in an environment where significant electromagnetic interference is present. Such shields are also used to keep internally-generated electromagnetic energy from being transmitted outwardly. Basic requirements for the shield are that it prevent significant energy penetration and that it provide a low-impedance path for the electrical current induced in it by the electromagnetic energy, whether that energy is generated externally or internally.
The most common electromagnetic shield is a continuous metal sheet enclosure. Although such an enclosure is effective in preventing externally-generated or internally-generated electromagnetic energy from passing through it, its weight and fabrication costs are considered to be excessive in many applications. Metal enclosures are also subject to corrosion in many environments. Only a small thickness is required for a solid metal shield to achieve the so-called skin effect, which causes the induced rf current to be conducted close to the shield's surface. However, the shield must ordinarily be substantially thicker than the skin depth to provide the desired structural strength. In addition, the joints between adjacent solid metal shields can sometimes allow leakage of electromagnetic energy. This is a particular problem when the amount of energy to be shielded against is large, or when low-level information signals are being conducted.
Efforts have been made in the past to reduce the weight and corrosiveness of metallic shielding by using a composite structure that includes bits of metal suspended in a predetermined relationship to each other in a lightweight, non-metallic binder material. Sufficient metal bits are included to ensure that they are in general contact with each other and thus provide a path for the electrical current induced by the electromagnetic energy. Although such composite structures are generally considered effective in shielding against some static forms of electromagnetic energy, they are not believed to prevent significant energy penetration, especially at high frequencies.
A special problem of the electromagnetic shielding occurs at the interface between two separate shields. This is of particular importance in electrical connectors between two overlapping, coaxial cylindrical conductors. The two cylindrical conductors typically have been electrically connected to each other by a coil spring wrapped into a toroidal shape and compressed between their two confronting surfaces. Such a shield coupling is generally considered effective at conducting electrical current from one cylindrical conductor to the other, with relatively low resistance and inductance. In addition, the spring's resilience ensures that contact is continuously made between the two conductors, even if they are not precisely concentric. It is believed, however, that the shielding provided by such a coil spring between two coaxial conductors can be further improved.